The object of the present invention is to provide a method and a device for the acquisition and the processing of signals obtained in, wells and notably in substantially horizontal deflected wells or well bores crossing a subterranean zone, allowing the good discrimination between the subsoil discontinuities. The invention is particularly adapted for discriminating between geologic discontinuities oriented substantially towards the same direction as the well and located on either side of the well, by means of recordings obtained with a logging tool displaced within the well. By what is called a deflected well is meant a well at least part of which is substantially horizontal or nearly horizontal.
Investigating a subsoil zone which might for example contain hydrocarbons can be achieved by drilling a well across the zone and by taking down within the well a logging tool comprising one or several emission units adapted for emitting signals in the geologic formations around the well and one or several reception units adapted for receiving the signals reflected and refracted by the subsoil discontinuities which are more or less close to the well. The received signals are generally transmitted to a surface laboratory and recorded. The recordings are then processed in order to make them more readable for the interpreter. The emitted and received signals can be of a very different nature. The signals may be, for example, acoustic waves, electromagnetic waves, etc. The method according to, the present invention is described hereafter in relation to the use of acoustic waves. It nevertheless goes without saying that this is a particular case which on no account restricts the generality of the method.
Well bores are often vertical or substantially vertical. But, in the subsoil zones which geologists think to be the most likely to contain workable deposits such as hydrocarbons, coal, lignite, etc, it is a common practice to carry out drillings that are very strongly inclined in relation to the vertical, even substantially horizontal, so that they cross the subterranean layers following a weak angle and information is obtained concerning the top and the wall (or base) of a possible working volume. Under these conditions, it is important to discriminate between the waves received from the formations above and below the well bore. This is possible as long as the dipping of the reflectors in relation to the direction of the well is not too weak and is of the same sign towards the top and the wall of the crossed zone or reservoir. If this is the case, the apparent velocities of the acoustic signals received (events) on either side of the well have opposite signs and they can be dissociated by using a conventional apparent velocity filtering method, as it is well-known in geophysics.
A prospecting method applicable in wells that are little inclined in relation to the horizontal is described for example in European patent application EP 246,148.
Discriminating on loggings of signals respectively coming from the wall and the top of a reservoir becomes very difficult when the relative dipping of the reflectors in relation to the direction of the well is almost nil or only amounts to some degrees because these signals have very close apparent velocities.
This configuration, which is frequent in strongly deflected or horizontal wells, can also be found in the vertical or little inclined parts of well bores crossing a fault zone substantially showing the same orientation. It is also well-known that the periphery of a well, whatever its direction may be, is generally weathered by the mud which has been used during the drilling process and which has infiltrated into the formations. The travel speed of the waves in this peripheral zone is therefore different from that in the surrounding formations, and parasitic reflections appear, therefore, on the interfaces that are substantially parallel to the direction of the well. Here again, discriminating on the recordings between the reflected waves respectively coming from the opposite sides of the well is almost impossible.
Published French patent application 2,639,719 describes a system for the acquisition and processing of signals in a well which makes it possible to achieve good discrimination between the geologic discontinuities located on either side of the well and having substantially the same orientation as it, notably in deflected wells crossing reservoir zones. The signals are acquired by means of a logging tool in which the emitting and receiving transducers can be displaced tranversely in relation to the well. This displacing can be obtained by using a logging tool comprising a body linked by moving arms to fastening skids against the walls of the well and by displacing the body in relation to the skids through the controlled opening of the arms. The displacing can also be obtained by displacing the transducers in relation to the body of the sonde. At least two series of emission-reception cycles are carried out along the same well portion, the position of the transducers in relation to the section of the well being different during each series, and the recording sections respectively obtained during the two series are combined in order to obtain sections where the discontinuities on either side of the well are separated from one another. The previous method is particularly adapted for discriminating and bringing out the discontinuities on either side of a well but it does not provide a sufficient damping of the refracted waves and of the waves known as the Stoneley waves.